Photocurable polymers and compositions are well known in the art for forming printing plates and other photosensitive or radiation sensitive articles. In the field of radiation sensitive flexographic printing plates, the plates typically comprise a support and a photosensitive surface or layer from a photocurable composition. Additional layers or surfaces on the plate include slip and release films to protect the photosensitive surface. Prior to processing the plate, the additional layers are removed, and the photosensitive surface is exposed to radiation in an imagewise fashion. The unexposed areas of the surface are then removed in developer baths.
Removal of unexposed surfaces comprising solid photocurable compositions such as those disclosed in U.S. Pat. No. 2,760,863 require the use of developer baths comprising environmentally unsafe, organic solvents such as tetrachloroethylene, 2-butanone, benzene, toluene, xylene, trichloroethane and solvent mixtures such as tetrachloroethylene/n-butanol. However, due to the toxicity, high volatility and low flash point, their use gives rise to hazardous conditions and creates pollution problems. Thus, recently there has been a strong interest in the field to develop photosensitive layers in non-organic solvent developing solutions, e.g., aqueous, surfactant-aqueous or alkaline-aqueous solutions. However, the compositions resulting from recent attempts to achieve aqueous developable plates demonstrate deficiencies in mechanical properties, e.g., flexibility. See European Application 261,910.
For instance, in addition to possessing an aqueous developable photosensitive surface, a flexographic printing plate must have sufficient flexibility to wrap around a printing cylinder, yet be strong enough to withstand the rigors experienced during typical printing processes. Further, the printing plate should be soft enough to facilitate ink transfer during printing.
Previous aqueous developable compositions have not possessed all the desirable features such as flexibility, softness and solvent resistance to inks typically used in printing. For example, U.S. Pat. No. 4,023,973 describes a photosensitive composition comprising a maleic anhydride adduct of a 1,2-polybutadiene. However, because the 1,2 content of this material is very high, i.e., 70% or more, this composition has an undesirably high rubber hardness.
Furthermore, other water-developable photosensitive compositions which contain as the main component a high molecular weight polymer such as polyvinyl alcohol, cellulose, polyethylene oxide, or the like, are insufficient in flexibility and possess a high degree of rubber hardness and hence are unsuitable for use in flexographic printing plates.
Finally, it is also important that the photosensitive surface of the printing plate be dimensionally stable during storage. For example, some compositions used for making plates have shown inferior stability properties when used in solid flexographic printing plates in that the compositions become tacky and pasty during storage. Those inferior properties have been attributed to the low molecular weight of the polymers used to prepare the printing plates. See U.S. Pat. No. 4,762,892 to Koch et al. and discussion of low molecular weight polymers disclosed in Japanese Kokoku 57-23693.
Photosensitive formulations using the polymers of this invention are water-developable without presenting any of the aforesaid disadvantages.
Japanese Kokai Patent (A), SHO 59-113430, Disclosure Date Jun. 30, 1984, Sakurai et al., discloses a water-developable printing plate made with a light-sensitive polymer. The polymer is made by chain extending both an isocyanate end capped polyester diol, and an isocyanate-hydroxyl(meth)acrylate adduct with an alkyl diethanolamine. Thus, a chain extended prepolymer terminated with isocyanate (such as Prepolymer III herein) is not made. The final polymer product is quaternized to increase cure speed. The present invention does not require this step. The solid photopolymer IV of the instant invention is not made. In Example 2 of the reference (which appears to be typical of the reference Examples), a mixture of an isocyanate end-capped polyether diol (PEG 1000) and a diisocyanate-hydroxyalkyl(meth)acrylate adduct is chain-extended with N-methyldiethanolamine under conditions leaving no detectable -NCO.
European Patent Application 54,150, filed Oct. 23, 1981, Lehner et al., assigned to BASF, describes a UV-activatable polymer made by first reacting together three components, i.e., a polyether diol, diisocyanate, and alkyldialkanolamine, and reacting that product with an epoxy acrylate. The final polymer product is formulated conventionally with acrylates and photosensitizers for use in water-developable printing plates. Since this reference describes a polymer comprising moieties of polyether diol, diisocyanate, alkyldialkanolamine, and acrylate, it is appropriate to consider it at length. In the Lehner et al. reference, the product is a random copolymer consisting of randomly positioned moieties of polyether polyol, diisocyanate, alkyldialkanolamine, and epoxy-acrylate in various combinations, e.g., B-A-B, B-C-B, B-A-B-C-B, B-A-B-C-B-D-B, etc., where A is polyether diol, B is diisocyanate, C is alkyldialkanolamine and D is an epoxy acrylate. This introduces pendant acrylate groups along the polymer chain. Finally, polymer viscosity is increased to obtain a solid polyurethaneurea by addition of a diamine which forms urea linkages. Lehner et al. did not react epoxy-acrylate sequentially. All A, B, C, and D were added at the same time. The Lehner et al. reference does not disclose an isocyanate end-capped polyether diol or polyester diol, chain extended with an alkyl dialkanolamine, with the chain-extended product further end-capped with hydroxyalkyl(meth)acrylate.
This invention differs in other important ways from the Lehner et al. process. (1) It is sequential, that is, the diol is end-capped, then chain-extended at a point in the process prior to terminating with acrylate. (2) The resulting molecule is then terminated with acrylate, whereas the reference reacts an epoxy-acrylate to form pendant groups along the polymer chain. (3) This invention does not utilize urea linkages. (4) This process does not quaternize with glacial acetic acid to make an ionomer. These differences ensure a precise and highly desirable thermoplastic polyurethane elastomer. That is, a structure that will provide the correct predetermined molecular weight and composition and will necessarily result in a printing plate with good physical properties and good aqueous and oil based ink resistance. The reference does not make the products of the instant invention.
U.S. Pat. No. 5,069,999, Higashi et al., "PS Plate for Making Lithographic Plate Requiring No Dampening Water," (1991), discloses a printing place comprising a photosensitive mixture comprising an acrylate monomer or oligomer and a polyurethane resin made ". . . by reacting a diisocyanate with approximately equimolar amount of a diol or those obtained by reacting a diol with a slight excess of a diisocyanate and then subjecting the reaction product to a chain extending reaction with a diamine, a diol, or water." [Emphasis supplied.] (Col. 4, line 66 - Col. 5, line 2.) The reference does not make a solid photopolymer, nor does it discuss or disclose water developability. Further, prepolymer III of the instant invention (a unique chain-extended polyurethane) is not disclosed in the reference. The reference suggests using alkyldialkanolamine or dihydroxy(meth)acrylate as a diol (Col. 5, lines 37-39). However, the reaction of a hydroxyalkyl-(meth)acrylate with a diisocyanate end-capped polyether diol further chain-extended with alkyldialkanolamine (Polymer IV of the instant invention) is not disclosed in the reference.
U.S. Pat. No. 4,888,379, Henning et al., "Heat-Sensitive Polyurethane Dispersions," (1989) discloses (Example 2) reacting a mixture of polycarbonate polyol and polyethylene oxide-polypropylene oxide polyether polyol with hexamethylene diisocyanate, followed by chain extending the resulting polyurethane with methyldiethanolamine (MDEA). The resulting material was further extended with isophorone diamine in water to form a polyurethane latex or emulsion useful for coatings. The final polyurethane emulsion is not photocurable.
U.S. Pat. No. 4,113,592, Rybny et al., "Trihalogenated Hydrocarbons as Co-Photoinitiators," (1978), teaches the addition of methyldiethanolamine to known photosensitizers to increase cure rate in photosensitizer compositions.
U.S. Pat. No. 4,980,108, Suzuki et al., "Process for Forming a Polyurethane Coated Biaxially Oriented Polyester Film," (1990) teaches introduction of pendant carboxylic groups into a polyurethane prepolymer followed by neutralization with methyl diethanolamine.
U.S. Pat. No. 4,003,751, Carder, C. H., "Coating and Ink Compositions," (1977), teaches reacting together a hydroxy(meth)acrylate, a monohydroxylamine, and an organic diisocyanate all in one step. The result is a mixture of polyurethanes formed from the hydroxy(meth)acrylate and hydroxylamine reacting separately with the diisocyanate. The reaction of a hydroxy(meth)acrylate with chain-extended isocyanate end-capped polyether and/or polyester diols is not disclosed.
U.S. Pat. No. 4,210,713, Sumiyoshi et al., "Photo-Curable Composition for Coating Containing an Unsaturated Urethane Modified Polymer," (1980), discloses a photopolymer made by reacting a diisocyanate with an acrylate. The acrylate can be the mono(meth)acrylic ester of N-methyldiethanolamine.
U.S. Pat. No. 3,864,133, Hisamatsu et al., "Photo-Polymerizable Compositions," (1975) discloses reacting, in specified sequence, a diisocyanate, a polyol, and an acrylate ester, optionally with an amine (e.g., methyldiethanolamine). The amine is said to accelerate hardening in the presence of air. In Example 6 tolylene diisocyanate was reacted with trimethylolpropane, and the result was reacted further with ethylacrylate and 2-hydroxyethylacrylate, followed by addition of triethanolamine. A mixture of acrylates was added as physical diluents to make the final photopolymer mixture. Example 49 discloses the reaction of tolylene diisocyanate with a polyester polyol, followed by reacting the resulting polyurethane with a mixture of methyl methacrylate and hydroxyethylmethacrylate. The result was mixed with triethanolamine. The sequential reaction of diisocyanate plus polyether and/or polyester diols, followed by chain extension with alkyldialkanolamine, followed by reacting with hydroxyalkyl(meth)acrylate is not disclosed.
U.S. Pat. No. 4,606,994, Illers et al., "Process for Producing Photo-cured Printing Plates Possessing a Defined Hardness," (1986) discloses reacting together a polyurethane with an acrylate to put acryloyl side groups on the polyurethane, optionally in the presence of dialkanolamine as chain extender. Prepolymer III and Photopolymer IV of the instant invention (as hereinafter described) are not made. Examples 1-3 describe polyether urethane possessing acrylate side groups prepared from polyethylene oxide polyol, bisphenol-A diglycidyl ether diacrylate, butanediol, cyclohexanedimethanol and hexamethylene diisocyanate. Chain extension of an isocyanate end-capped polyether or polyesterdiol with an alkyldialkanolamine followed by reaction with a hydroxyalkyl(meth)acrylate is not disclosed. Furthermore, the resulting photosensitive plates were developed in alcohol/water mixtures, which are less environmentally safe than this invention which uses dilute solutions of water and surfactants, vinegar or citric acid.
U.S. Pat. No. 4,716,094, Minonishi et al., "Photosensitive Resin Composition Which is Improved with Respect to Surface Tack-Free Characteristic after Curing, and a Method," (1987) suggests (Col. 5) reacting an isocyanate end-capped polyether polyol with a compound having a hydroxyl, carboxyl, or amino group containing unsaturated carboxylic acids or esters to prepare a printing plate prepolymer.
Japanese Kokai Patent, HEI 2-1859, disclosure date, Jan. 8, 1990, Kawahara et al., "Photosensitive Resin Composition," discloses making a photopolymer with a softening point of 45.degree.-80.degree. C. to enhance water developability. This is disclosed to be accomplished by reacting a diisocyanate-capped polyether diol with hydroxyethylmethacrylate. Chain extension with alkyldialkanolamine to form a thermoplastic elastomer polyurethane is not used in the process.
Japanese Kokai Patent (A), HEI 2-4255, of Jan. 9, 1990, Satomi et al., "Light-Sensitive Composition," discloses preparation and use of a urethane acrylate polymer similar to that of Japanese Kokai Patent HEI 2-1859, above, except that a polymeric material is added mechanically to regulate elasticity and hardness after photoexposure.
European Patent 0 367 086 of Jan. 25, 1989, Littmann et al., assigned to BASF AG, discloses a water developable printing plate using a carboxylated polymer with acrylate-terminated chains. Diisocyanate is not used.
European Patent Application, Publication No. 0 351 628 A2, of Jul. 4, 1989, Gersdorf et al., "Photosetting Elastomer Mixture and Recording Material Obtained from It for the Production of Relief Printing Plates," assigned to Hoechst AG, discloses a polyurethane base polymer grafted with a vinyl carboxylate which is later saponified to form acid groups and admixed with acrylates and photosensitizer to make a water-developable printing plate. There are no alkyldialkanolamine or acrylate groups in the base polyurethane chain.
In European Patent 374,707 of Dec. 13, 1989, Gersdorf et al., "Photocurable Elastomer Mixture Containing Polyurethane, Unsaturated Monomer and Initiator, Useful for Printing Plate and Photoresist Production," assigned to Hoechst AG, the mixture formulated for printing plates comprises polyurethane grafted with a vinyl carboxylate, plus acrylate diluent and photosensitizer. The exposed plate is water-developable.
Japanese Kokai Patent (A), HEI 2-46460, of Feb. 15, 1990, Tomita et al., "Light Sensitive Resin Composition," discloses a polyurethane prepolymer treated with dimethylolpropionic acid to yield a further polyether urethane, which, with other polyether urethanes, is mixed with acrylate and photosensitizer in the preparation of a water-developable plate. The base photopolymer molecule did not contain either alkyldialkanolamine or acrylate.
Xiao, H. X., et al., "Urethane Ionomers," 32nd Annual Polyurethane Technical Marketing Conference, Oct. 1-4, 1989, pp. 398-411, discloses reacting diisocyanate with polyether diol, followed by reaction with methyldiethanolamine until the mixture showed zero cyanate. The resulting chain-extended prepolymer thus had no molecules terminated by isocyanate, as required in Prepolymer III of the instant invention, and the final polyurethane does not contain any photocurable sites.
Speckhard, T. A., et al., "Properties of Segmented Polyurethane Zwitterionomer Elastomers," J. Macromol. Sci.-Phys., B 23 (2), 175-199 (1984), discloses (Scheme 1, p. 178), the reaction of methyldiethanolamine directly with isocyanate-capped polyether diol in the absence of free diisocyanate. No polyurethane prepolymer of the type diisocyanate/methyldiethanolamine/diisocyanate is made, as required in the instant invention, and the product as chain-extended with methyldiethanolamine is shown as terminating in hydroxyl, not isocyanate. And there is no acrylate terminator. The final polymer is not a photopolymer.
Show-an Chen, et al., "Polyurethane Cationomers, I. Structure-Property Relationships," Journal of Polymer Science, Part B: Polymer Physics, Vol. 28, 1499-1514 (1990), discloses (p. 1500) reacting polyether diol with an excess of toluene diisocyanate, followed by reaction with methyldiethanolamine, until the reaction mixture tests zero isocyanate, thus indicating absence of isocyanate terminated molecules. (The instant invention requires isocyanate terminated prepolymer at this stage, defined as Prepolymer III. There are no acrylate groups in the reference polymer.
Miller, J. A., et al., "Properties of Polyether-Polyurethane Anionomers," J. Macromol Sci.-Phys., B 22 (2), 321-341 (1983). This reference discloses (p. 325) end-capping polyether polyol with an excess of diisocyanate, followed by chain extension with methyldiethanolamine. The latter reaction is continued until the reaction mixture shows zero isocyanate, thereby forming a prepolymer without terminal isocyanate groups. (At this stage the instant invention requires isocyanate terminated prepolymer Prepolymer III, as hereinafter defined.)
Hsieh, K. H., et al., "Effect of Charge Groups in Polyurethane Ionomers and Blends," Adv. Urethane Sci. Technol., vol. 10, pp. 77-83 (1987). Page 79 discloses casting a physical mixture of isocyanate capped polyether polyol and methyldiethanolamine in a mold at 70.degree. C.
Hsu, S. L., et al., "Polyurethane Ionomers. I. Structure-Properties; Relationships of Polyurethane Ionomers," Journal of Applied Polymer Science, Vol. 29, 2467-2479 (1984), describes end-capping polyether diol with diisocyanate, followed by reaction with methyldiethanolamine. The reaction was stopped when the reaction mixture showed zero isocyanate (p. 2468), indicating that the resulting prepolymer was not terminated with isocyanates. (At this stage the instant invention requires isocyanate terminated prepolymer, Prepolymer III as hereinafter defined.) Preparation of a photocurable polymer with acrylate groups was not disclosed.
Encyclopedia of Polymer Science and Engineering, John Wiley & Sons (1988), 2d Ed., Vol. 13, pp. 259-261, discloses the general reaction of isocyanates with polyester polyols to form polyurethanes.